In her tweets, Mrs Buhari said that the president was continuing to "carry out his responsibilities" and has been meeting with ministers.

Buhari's absence fuels speculation, Stephanie Hegarty, BBC Africa:

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President Muhammadu Buhari was in the UK for seven weeks for treatment

Mr Buhari has rarely been seen outside of his official residence in the eight weeks since he returned from the UK.

But the real controversy began when he stopped attending an important weekly cabinet meeting four weeks ago.

#WhereisBuhari started trending online and the writer Wole Soyinka, as well as prominent civil society activists, is calling on the president to release his medical records and to take official medical leave once again.

This is a crucial year for Nigeria and important decisions need to be made as the country struggles to get out of recession.

The presidency is keen to assure the public that Mr Buhari is still very much in charge because a power vacuum, or even the perception of it, could have very damaging effects.

Vice-President Yemi Osinbajo, who was the acting president while Mr Buhari was away in the UK, chaired the cabinet meeting in the capital, Abuja.

On Monday, a group of prominent Nigerians has called on Mr Buhari to take medical leave as concerns about his health grow.

They said in a statement that the president's absence from the cabinet meetings, as well as the weekly Friday Muslim prayers, "has fuelled further speculation and rumours" about his medical condition.

The 13 said they felt "compelled" to ask Mr Buhari "to heed the advice of his personal physicians by taking a rest to attend to his health without any further delay".